1. Field of the Invention
The present invention relates to a power supply circuit for supplying power to an electronic device such as an image forming apparatus.
2. Description of the Related Art
A developing device in an electrophotographic or electrostatic printing image forming apparatus efficiently develops an electrostatic latent image with toner by applying, to a developing sleeve, a voltage in which DC (Direct Current) and AC (Alternating Current) voltages are superposed. In particular, an AC voltage having a rectangular waveform increases the charging efficiency of toner with respect to a latent image (ratio at which toner charges couple with latent image charges).
A voltage applied to the developing sleeve needs to have a target value. This is because various problems arise if an applied voltage greatly exceeds a target voltage to generate an overshoot. For example, a current flows through an unintended conductor via the surface of an insulator or an air layer. Also, aerial discharge occurs in a conductive impurity mixed in a developing agent, damaging a latent image. As one solution to relieve these problems, a damping resistor having a sufficiently large resistance value is adopted.
However, the use of the damping resistor also has disadvantages. For example, the rise and fall responses become slower than those of an ideal rectangular wave, resulting in a blunt rectangular wave. The blunt rectangular wave is poorer in charging efficiency than the ideal rectangular wave. The power loss across the damping resistor accounts for half the input power to an AC voltage generation circuit, increasing the space for permitting the energy loss and raising the component cost.
In Japanese Patent Laid-Open No. 2002-354831, an AC voltage generation circuit is formed from a full bridge circuit made up of four switching elements. A predetermined OFF period is set in part of a period during which each switching element is turned on. This arrangement relieves an overshoot without depending on a damping resistor.
The invention disclosed in Japanese Patent Laid-Open No. 2002-354831 can correct an output waveform distortion by LC resonance satisfactorily for a developing unit, photosensitive member, developing high-voltage power supply, and developing agent under a given condition. However, a problem arises when the conventional technique is applied to a so-called blank pulse waveform having a pulse period during which a rectangular wave is output and a blank period during which output of a rectangular wave stops.
FIG. 8 exemplifies a waveform when the conventional technique is applied to the blank pulse waveform. When a constant OFF period is always used, like the conventional technique, LC resonant waveforms appear at a leading edge from the blank period to the pulse period and a trailing edge from the pulse period to the blank period. Such a waveform distortion degrades the developing quality, as described above.